The invention relates to a lost foam casting system, and more particularly to an improved gating sprue.
Lost foam casting is a known technique. A pattern assembly formed of evaporative foam material, for example gasifiable or liquifiable expanded polystyrene, which may be coated with a synthetic resin, or for example polymethylmethacrylate (PMMA), is placed in a flask and surrounded by unbonded particulate media, most commonly sand, though other particles can be used such as zirconia, magnetic iron spheres, etc. Upon application of molten metal to the pattern assembly, the foam material vaporizes and is replaced by the metal in the shape of the pattern assembly. The vaporized foam material escapes into the interstices in the sand. The pattern assembly includes one or more workpieces of given respective patterns for yielding cast metal parts, and a sprue connected by one or more in-gates to the workpieces for communicating the molten metal through the sprue to the in-gates and then to the workpieces. After cooling, the cast metal sprue and in-gates are broken away from the workpieces and discarded or recycled. The cast metal work pieces are then trimmed to yield the final product.
The present invention arose out of efforts to provide improved cast aluminum alloy cylinder heads for internal combustion engines in marine applications, though the invention is not limited thereto. Cylinder heads have a fairly complex pattern, and the fluid dynamics of the molten metal flow become critical because the flow must reach the extremities of the cylinder head pattern before solidification. Another particular requirement in casting cylinder heads is extremely tight dimensional repeatability in order to maintain precise dimensions and volumes of the internal combustion chambers. Too great a tolerance deviation will vary the compression ratio of the engine cylinders.
Pattern assembly rigidity is desired to maintain shape and dimensional integrity during the liquification cycle when molten metal is applied to the pattern assembly. The inherent mechanical strength of foam material is low and hence the pattern assembly must be supported or otherwise bolstered to maintain sufficient stiffness during the liquification cycle in order to prevent distortion. It is known in the prior art to wrap straps of polystyrene around the pattern assembly or to use extremely large thick stock sprues for the purpose of giving dimensional stability and stiffness. This approach suffers the disadvantage of wasting large amounts of cast metal in the rigging or sprue portion of the assembly for the sole purpose of adding rigidity, and which metal is not part of the cylinder head or other workpiece. This increased stock and thickness approach also requires longer cooling time, and furthermore adds to the amount of vaporized material which must be absorbed into the interstices in the sand. A further disadvantage is that the increased foam material adds to buoyancy during pattern processing and pattern investing and during the liquification cycle.
The present invention provides a relatively rigid pattern assembly without the above noted disadvantages. The invention provides rigidity while at the same time providing high casting yields, i.e. high ratio of workpiece cast metal weight to sprue cast metal weight. The invention further provides a high surface area to volume ratio to permit escape of organic effluents and to promote rapid cooling. The invention provides low total volume of the sprue to conserve metal, and to reduce buoyancy during liquification, and hence reduce lift. The invention further promotes uniform filling at the in-gates, to facilitate complex pattern casting. The invention further facilitates ease of workpiece pattern attachment to the sprue, and accessibility of the cast workpieces for ease in removal from the sprue after solidification.
The high surface area sand contact to volume ratio facilitates vaporization of the foam material which in turn speeds the process. This is particularly desirable because it enables faster fill rates, which is desirable in complex patterns, or which may enable certain patterns not otherwise feasible.
The invention further facilitates noncritical placement of the in-gates and/or the workpieces to the sprue. The sprue is provided with multiple internal feed passages and with large area flat external sides affording noncritical placement of such in-gates and/or workpieces thereto, which flatness is maintained during the liquification cycle. The sprue provides a nonflexing repeatable affixation surface without increasing bulk density of the sprue.
In the preferred embodiment, the invention facilitates minimization of the number of glue joints on the pattern assembly. The invention enables a one-piece sprue. The workpieces are integrally formed with the in-gates, and the in-gates are glued to the sprue. The one-piece sprue and the minimal number of glue joints is a significant accomplishment. Prior assemblies have required numerous glue joints. A high number of glue joints is undesirable because of the high labor cost in the various attachments which must be made and because of the increased organic material which must be displaced and absorbed into the sand. This increased organic material may cause voids or defects in the final cast parts, and hence degrade quality.
In a further desirable aspect of the invention, the sprue is made in a mold splitting the sprue pattern diagonally such that any rough edges in the sprue pattern upon opening of the mold halves are formed at diagonal edges and not along flat affixation surfaces. Additionally, sand fill apertures are formed through flat support walls in the sprue between feed passages, which apertures are beveled to facilitate opening of the mold.
The preferred sprue further includes internal structure causing a draft during sand filling, facilitating full sand fill in and around the sprue, to provide maximum support and sand surface area contact.